There are two major types of screw and nut mechanisms: a first type utilizes a popular Acme lead screw while a second type utilizes a ball screw. Both of these screw types are generally known to operate with inefficiency due to backlash effects that generally arise in rotatable mechanical components. Although attempts have been made to alleviate this condition, there is still much room for improvement.
In the prior art, the first type of screw—the Acme screw—has difficulty maintaining efficient rotation without pitch deviation and/or backlash, but these difficulties are somewhat easier to overcome than in the second type of screw, since there is not the complication engendered by a quantity of rolling balls that also need to be controlled. The Acme lead screw, however, is considered to be less efficient than any ball screws.
Acme lead screw and nut devices generally suffer from a very low efficiency which is around 50%, as compared to ball screw and nut devices. The prior art anti-backlash screw and nut devices for Acme lead screws are very bulky and large in volume.
Ball screws which are used to convert rotary input into linear output motion are considerably more efficient and more accurate than Acme lead screws and many other kinds of actuators, such as a belt, a cable, chain drives, and the like. For this reason, therefore, they are extensively used in many applications: in the automobile industry, in electronic equipment, in engineering machinery, in CNC machine tools, in the field of automation, in the railroad industry, in medical equipment, and in many others.
In the prior art, the second type of screw—the ball screw—is used in screw and nut mechanisms where the screw and nut are provided with matching helical grooves or races which are used to retain a plurality of rolling balls and allow them to roll in these grooves so as to load the screw and nut assembly. Each of the balls generally provides dual points of contact being positioned between the screw and nut, although the mechanism in some prior art configurations can be made to increase the number of contact points further in order to increase the load capacity of the mechanism for heavy duty applications (see FIG. 3).
For ball screws, there are generally two types of ball return systems in the prior art. In the first type, the balls are returned to a starting point in the active circle—i.e., the path where the balls provide the load which is disposed external to the outside diameter of the ball nut (see U.S. Pat. No. 2,855,791 to W. H. Hogan as shown by way of example in FIG. 1 of the present invention).
In the second type of ball return system, the balls are returned in an internal pathway provided inside the diameter of the outer wall of the nut (see example from Barnes Industries shown in FIG. 2). There are several alternate return systems to the second type of ball return system. For example, the balls make one helical circuit around the ball screw in what is called an active circle and then are made to cross-over into an adjacent groove above the outside diameter of an adjacent thread to return to the active circle for the next cycle of activity. This is called a cross-over or “flop over” design (Rotex of England) internal return system. A deflecting means, such as a tab deflector supported by the nut is used to redirect the balls along the return pathway. Another alternate return system is called the tangential internal ball return system (Barnes Industries). A V-cap is used to return the balls to the active circle on an opposite side of the circuit inside a bore just below the outer wall of the nut.
Ball screws are often manufactured with a one-start, two-starts, or multi-start screw. This refers to the number of independent threads on the shaft of the screw. In some applications, additional series of rolling balls accommodated in the multiple pathways of multi-start screws are used to increase the load linear speed of the ball screw and nut where a large pitch is needed for the ball screw for higher speed applications.
Ball screws are subject to degradation and wear from many causes as in most mechanical systems, but they generally last much longer than simpler Acme screw and nut mechanisms and can operate at higher speeds and on heavier loads. Nevertheless, prior art screw and nut assemblies are very sensitive to temperature change, as well as to problems from thread profile and pitch deviation, the latter problem generally occurring relatively frequently in large volume production.
Both prior art Acme lead screws and ball screws suffer from backlash—the free axial movement of the central screw in the former type of screw assembly, and the motion of balls along the screw threads in the latter. Most backlash effects occur between the screw and nut.
It is important to optimize performance by eliminating or at least minimizing this backlash. One way to do this, as is known to those skilled in the prior art, is to preload the nut. The nut is loaded so as to apply pressure on the screw threads in the direction opposite the working load and without allowing freedom of movement in between the screw and the nut. There are several ways that this is done as will be described hereinafter in the detailed description of the prior art in reference to FIG. 3.
Prior-art ball screws, those with an external return tube, are designed having a minimum 540 degree active ball circle (i.e., 360+180) comprising movement along one and a half threads. On the other hand, those prior-art ball screws with an internal return pathway have only about a 300 degree active ball circuit. It is important to control the rolling pathway for the balls that helically encircle a ball screw fitted with a complementary nut so that unnecessary rolling motion does not cause asymmetrical inefficiency in the active circle.
Accordingly, it is a broad object of the present invention to overcome the above disadvantages and limitations of the prior art by providing an anti-backlash device and a method suitable for use with both an Acme lead screw and a conventional ball screw.
It is another object of the present invention to provide an anti-backlash device wherein only one face of a cradle thread is required to be accurately machined, whereas the other components may be made of relatively less accurate machined parts.
Still another object of the invention is to provide a full, helical, active circle of about 360-degrees for a plurality of shaped rollers in order to provide a symmetrical balanced load on the roller screw component utilizing an anti-backlash device.
Yet another object of the present invention is to provide a full, helical, returning circuit of about 360-degrees for a plurality of shaped rollers.
Still another object of the present invention is to provide a compact, lightweight and less bulky anti-backlash device.
A further object of the present invention is to provide a low-cost and low-friction anti-backlash device.
Therefore there is provided an anti-backlash device for preventing backlash in a screw and nut mechanism having helical threads and used in converting rotary motion into linear motion, the anti-backlash device comprising:
a cylindrical, pressure actuator mounted around the screw and integrally formed with a helical thread; and
a cylindrical cradle integrally formed with a helical, internal thread for meshing with the helical screw threads, the cradle being mounted around the screw exterior to and in close proximity to the pressure actuator,
wherein both the pressure actuator and cradle each have only one accurately machined face axially oriented inward toward the screw, and
wherein when the screw is operated to rotate within the cradle, the helical, pressure actuator thread is loaded on the helical screw thread so as to exert pressure thereon in a first axial direction, while simultaneously pressure is applied in an opposing, second axial direction by the cradle internal thread when loaded on the helical screw thread thereby applying a predetermined, axial, balanced force on the screw so as to prevent backlash.